Explicitly control lifetime of memory owner

src/ImageSharp/Formats/Png/PngEncoderCore.cs

@@ -680,13 +680,16 @@ namespace SixLabors.ImageSharp.Formats.Png
                 return;
             }
 
-            using IMemoryOwner<byte> owner = this.memoryAllocator.Allocate<byte>(xmpData.Length + PngConstants.XmpKeyword.Length + 5);
+            int payloadLength = xmpData.Length + PngConstants.XmpKeyword.Length + 5;
+            using (IMemoryOwner<byte> owner = this.memoryAllocator.Allocate<byte>(payloadLength))
+            {
                 Span<byte> payload = owner.GetSpan();
                 PngConstants.XmpKeyword.CopyTo(payload);
                 int bytesWritten = PngConstants.XmpKeyword.Length + iTxtHeaderSize;
                 xmpData.CopyTo(payload.Slice(bytesWritten));
                 this.WriteChunk(stream, PngChunkType.InternationalText, payload);
             }
+        }
 
         /// <summary>
         /// Writes a text chunk to the stream. Can be either a tTXt, iTXt or zTXt chunk,